The replacement of incandescent bulbs by light emitting diode (LED) lighting devices reduces energy consumption due to the high efficiency of LED devices as compared to incandescent bulbs. But an incandescent bulb may be directly driven by the AC mains in contrast to conventional LED devices. It is thus conventional for an LED device to include a switching power converter such as a flyback converter to provide a regulated DC current for driving the LED. This need for a switching power converter raises costs and thus diminishes a consumer's desire to switch to LED devices. A consumer may thus continue to use incandescent bulbs, which contributes to global warming due to the increased greenhouse gas emissions from the resulting energy consumption.
To lower LED device costs, direct AC LED devices (which may also be denoted as direct AC LED bulbs) have been developed that obviate the need for a switching power converter. In a direct AC LED device, the AC mains voltage is rectified through a rectifier such as a bridge diode rectifier to produce a rectified AC input voltage. The LED in a direct AC LED device is directly driven by the rectified AC input voltage. Although no switching power converter is thus needed to convert the rectified AC input voltage into a regulated DC current/voltage, a direct AC LED device still needs a controller to control the LED power. In particular, the controller controls an LED current source in series with the LED. When the rectified AC input voltage (which may also be denoted as a post diode bridge voltage) rises above the LED threshold voltage for the LED, the controller controls the LED current by controlling the LED current source so that the LED power may be controlled accordingly.
The presence of a phase-cut dimmer switch in household applications complicates the control of direct AC LED lighting devices. In particular, a phase cut dimmer's TRIAC requires a minimum amount of holding current when conducting to prevent the TRIAC from resetting. But the post diode bridge voltage may not have exceeded the LED threshold voltage when the TRIAC begins conducting. A direct AC LED lighting device 100 that is compatible with phase-cut dimming applications thus will typically include a bleeder circuit 110 as shown in FIG. 1. A dimmer switch is represented by a TRIAC that intervenes between an AC mains (AC_Input) and a diode bridge (DB). The diode bridge rectifies a phase-cut AC input from the TRIAC to produce the post diode bridge voltage carried on a power rail 105. Bleeder circuit 110 couples to power rail 105 to conduct a holding current into ground. A controller (I_CNTRL) controls a current through an LED string by controlling an LED current source 115.
The dimmer switch has an internal RC circuit (not illustrated) that controls its firing time in each AC half cycle. But this firing time is subject to change due to variations in DIAC threshold voltage and RC integration current for the dimmer switch. The firing time is thus typically subject to 50 to 100 μs of jitter from a given AC half cycle to a subsequent AC half cycle. The resulting jitter is shown in FIG. 2 for a leading edge phase cut dimmer. In each AC half cycle, the post diode bridge voltage rises above the LED threshold voltage so that the LED current can flow. But the TRIAC firing time for a subsequent AC half cycle 205 is delayed as compared to the TRIAC firing time for an initial AC half cycle 200 by the typical dimmer switch jitter of 50 to 100 μs. This jitter is significant enough to cause random light intensity changes that are detectable by the human eye. The resulting rapid variation in the LED illumination is disconcerting and discourages users to switch from incandescent lighting to direct AC LED devices.
Accordingly, there is a need in the art for direct AC LED devices with reduced jitter in the presence of a phase-cut dimmer switch.